What if the way you live now affects the life expectancy of your children and grandchildren?

Take, to begin with, the Swedish chickens. Three years ago, researchers led by a professor at the university of Linköping in Sweden created a henhouse that was specially designed to make its chicken occupants feel stressed. The lighting was manipulated to make the rhythms of night and day unpredictable, so the chickens lost track of when to eat or roost. Unsurprisingly, perhaps, they showed a significant decrease in their ability to learn how to find food hidden in a maze.

The surprising part is what happened next: the chickens were moved back to a non-stressful environment, where they conceived and hatched chicks who were raised without stress – and yet these chicks, too, demonstrated unexpectedly poor skills at finding food in a maze. They appeared to have inherited a problem that had been induced in their mothers through the environment. Further research established that the inherited change had altered the chicks’ “gene expression” – the way certain genes are turned “on” or “off”, bestowing any given animal with specific traits. The stress had affected the mother hens on a genetic level, and they had passed it on to their offspring.

The Swedish chicken study was one of several recent breakthroughs in the youthful field of epigenetics, which primarily studies the epigenome, the protective package of proteins around which genetic material – strands of DNA – is wrapped. The epigenome plays a crucial role in determining which genes actually express themselves in a creature’s traits: in effect, it switches certain genes on or off, or turns them up or down in intensity. It isn’t news that the environment can alter the epigenome; what’s news is that those changes can be inherited. And this doesn’t, of course, apply only to chickens: some of the most striking findings come from research involving humans.

One study, again from Sweden, looked at lifespans in Norrbotten, the country’s northernmost province, where harvests are usually sparse but occasionally overflowing, meaning that, historically, children sometimes grew up with wildly varying food intake from one year to the next. A single period of extreme overeating in the midst of the usual short supply, researchers found, could cause a man’s grandsons to die an average of 32 years earlier than if his childhood food intake had been steadier. Your own eating patterns, this implies, may affect your grandchildren’s lifespans, years before your grandchildren – or even your children – are a twinkle in anybody’s eye.

It might not be immediately obvious why this has such profound implications for evolution. In the way it’s generally understood, the whole point of natural selection – the so-called “modern synthesis” of Darwin’s theories with subsequent discoveries about genes – is its beautiful, breathtaking, devastating simplicity. In each generation, genes undergo random mutations, making offspring subtly different from their parents; those mutations that enhance an organism’s abilities to thrive and reproduce in its own particular environment will tend to spread through populations, while those that make successful breeding less likely will eventually peter out.

As years of bestselling books by Dawkins, Daniel Dennett and others have seeped into the culture, we’ve come to understand that the awesome power of natural selection – frequently referred to as the best idea in the history of science – lies in the sheer elegance of the way such simple principles have generated the unbelievable complexities of life. From two elementary notions – random mutation, and the filtering power of the environment – have emerged, over millennia, such marvels as eyes, the wings of birds and the human brain.

Yet epigenetics suggests this isn’t the whole story. If what happens to you during your lifetime – living in a stress-inducing henhouse, say, or overeating in northern Sweden – can affect how your genes express themselves in future generations, the absolutely simple version of natural selection begins to look questionable. Rather than genes simply “offering up” a random smorgasbord of traits in each new generation, which then either prove suited or unsuited to the environment, it seems that the environment plays a role in creating those traits in future generations, if only in a short-term and reversible way. You begin to feel slightly sorry for the much-mocked pre-Darwinian zoologist Jean-Baptiste Lamarck, whose own version of evolution held, most famously, that giraffes have long necks because their ancestors were “obliged to browse on the leaves of trees and to make constant efforts to reach them”. As a matter of natural history, he probably wasn’t right about how giraffes’ necks came to be so long. But Lamarck was scorned for a much more general apparent mistake: the idea that lifestyle might be able to influence heredity. “Today,” notes David Shenk, “any high school student knows that genes are passed on unchanged from parent to child, and to the next generation and the next. Lifestyle cannot alter heredity. Except now it turns out that it can . . .”

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